There is no simple "reliable" way: Try This – as it works for us:
Please note: It is CRITICAL to avoid any downloads of new mail while you are working on this migration… until you have imported all your old (exported) mail! CRITICAL… or you may screw it all up.
A: Go to Outlook and right-click your Outlook Today’s <Personal Folders> item the Folder List panel, then choose “Properties” for “Personal Folders”. It is usually headed, “Personal Folders – Outlook Today” Figure 1: Opening the Properties window.
NOTE: IF YOUR FOLDERS LIST IS HIDDEN, OPEN THE VIEW MENU AND CHOOSE FOLDER LIST.
B. Go to the Personal Folder Propertied wind, then select the ADVANCED button by clicking on it. Right Click on the inside of the path, and select COPY.
C. Go to the Start Menue and point to Programs (All Programs) where you point to Accessories. Then Click on Notepad.
D. While in Notepad, you must open the EDIT menu, then select PASTE to paste the folder path into Notepad at that window.
E. At the end of the PATH, you should find the folder name and the extension called: PST or pst. You must delete the end of the path until the last character is the BACKSLASH ( \ )
This may be listed as “Untitled Notepad, with the box where you will have choices “File, Edit, Format, View, and help”
C:\Documents and Settings\Name ?\Local Settings\Application Data\Microsoft\Outlook\
See the Path after deleting. Notice that the folder name and extension are gone.

D:\ Depress Control A to select the entire modified path and depress Control-C to copy it to the clipboard.
E:\ Open the Start menu again to choose Run… Depress Control-V to paste the path into the Run field.
Then click OK
F:\ Windows should open the folder containing Outlook’s data files. Copy all of those files to your new computer.
If you have been using Mail Rules in Outlook, continue to the next section. If not, then you should skip to the Outlook XP Import.
You should follow these Outlook Mail Rules (if you use mail rules. If not, skip this step
The Mail Rules are used to automatically sort incoming or outgoing e-mail based on rules that the user can define in the Rules Wizard. If you want to move these to your new computer along with your Outlook data, follow these steps:
AA. Open the Tools menu in Outlook and choose the Rules wizard
BB. At the bottom of the Rules Wizard screen, click on the Options button.
CC. Click on the Export Rules, then type a file name for the exported Rules File, and choose Save. You must copy this file to your new computer.

XP Outlook Import (Outlook Data):
This importing is very familiar to the Exporting in XP Outlook above:
aa. Follow the steps in the Outlook 2000/XP Export section, up to step 8.
bb. Close Outlook by opening the File menu and selecting Exit. Keep this folder Window that contains the Outlook js data files opened.
cc: Delete the files in the data folder.
As mentioned before, this description of procedures assumes you have NOT received any mail (i.e. Send/Receive) on your new computer. If you have received or sent , you must use the IMPORT and EXPORT wizard so you do not lose your new mail. Using the wizard is a more difficult way to import your mail and will not import your Auto Archive files.
1. Now, copy your exported data files into the folder window from your backup medium. Par example, if you copied the Outlook data files from your old computer onto a recordable CD, insert that CD and copy the files into the data folder.
2. Windows will ask you if you want to overwrite the existing files. Here, you select Yes.

Now, with any luck at all you! You have finished the import/export process! You can continue on to Outlook Mail Ruels, or skip to the end

Outlook Mail Rules:
If you exported any mail rules on your old computer, you can follow these steps to import them into Outlook on your new computer:
001. While in Outlook, open the the Tools menu and choose Rules Wizard.
002. At the bottom of the Rules Wizard dialog, select the Options button.
003. Click Import Rulesj icon, and select the Rules files you exported in the Export Section

Your Address Book:
Follow these steps to export your Outlook Express Address Book:
If you are using Windows XP:
Aa. Open the Start menu and select Run…
Bb. Enter the path: C:\Documents and Settings into the Run box and click OK.
Cc. Double-click on the folder with your user name on it.
Dd. Double-click on the Application Data folder.
Take note: If your Application Data folder is NOT visible, open the Tools menu and choose Folder options. Switch to your View tab and set Hidden files and folders to Show hidden files and folders under Advanced Settings.

On the one hand, it is a difficult program because it requires a lot of protection to be really safe.
On the other hand, they want you able to migrate the files so you will be willing to buy the Office upgrades...

There is also software, which costs about $29, that will move everything with nary a problem.

If that fails, you may also try the "file and settings transfer wizard (I believe on Vista is 'Migration wizard').
You'll need to run it on your Vista machine first, then put the program on a flash drive and select your Office settings (or anything else for that matter) on your XP machine and your Vista computer will have everything as you had it on your XP.

I am only a rookie, and I have done this using import/export in the past, I am wondering why the Migration wizard wouldn't be the preferred method, and that way, you could also import any other programs / files at the same time?

It can be... if you have no special setups... and it is not a corporate machine or one with a proprietary security arrangement.
We use the slow, plodding system, because in our part of the world, Outlook is used for specific purposes, and too often security is installed which blocks the migration attempts... and often the person attempting the migration does not even know... then problems all around.

My guess is that an Outlook user who does like outlook would never want to transfer Outlook messages to Thunderbird. Outlook is a major choice of businesses and agencies who need to keep track of all messages, dates, etc., and when combined with Microsoft's Business Contract Manager, is a major power in communications which must be tracked and filed.
There are really few substitutes that keep organizations as legal and reliable as Outlook, without paying for extra costs an incurring the wrath of oversite agencies and laws.
This is why transferring messages appropriately is a necessary evil.
Thunderbird is excellent, but is easily "edited"
We find that people who really know how to use Outlook like it very much, and cannot do without it... and those people do not appreciate Thunderbird.